This is the smallest of three similar-looking species most commonly-referred to as the 'bleeding heart' subgroup, the other two being H. erythrostigma and H. socolofi.
All possess a reddish humeral spot which is not present in any other characid, with other shared characters including possession of 6 -14 maxillary teeth, 7-9 scales above the lateral line, 5-7 scales below the lateral line and 26-33 anal fin rays.
H. pyrrhonotus can be told apart from the other bleeding heart tetras quite easily since it uniquely possesses a red, wedge-shaped patch of red pigmentation in the upper part of the body, extending from below the dorsal-fin to the base of the caudal. This may be more apparent in well-conditioned or dominant individuals, sometimes reaching the head and snout in sexually mature males.
It is further distinguished from H. erythrostigma by lacking hook-like structures in the fins of mature males (vs. hooks present in the anal fins, dorsal and pelvic in mature males of H. erythrostigma), and from H. socolofi by the following combination of characters: 33-34 vertebrae, mode 33 (vs. 31-33, mode 32 in H. socolofi); a white longitudinal band extending distally along the majority of the anal-fin in adults, covering 70 to 95 % of the rays (vs. white band covering 45-50 % of the branched anal-fin rays only); first branched ray of anal-fin longest in adult males (vs. third branched ray); length of anal-fin in adult females 21.8-24.5 % SL, mean 23.52 % (vs. 18.01- 21.22 % SL, mean 19.37 %); no hook-like structures on fins (vs. numerous hooks on all fins except the adipose -fin in mature males and anal-fin in mature females); scales cycloid (vs. scales cyclo-ctenoid in mature males and some mature females).
H. pyrrhonotus also has the most restricted range among the bleeding heart group with H. erythrostigma relatively widespread in the upper Amazon region from the rio Purus, Brazil, at least as far upstream as the Río Nanay in Peru, and H. socolofi occurring throughout much of the rio Negro basin, including the rio Branco, plus the rio Nhamundá which enters the Amazon downstream of its confluence with the Negro.
Limited genetic analyses have revealed that the bleeding heart group is probably monophyletic, but that there may be more species involved (Paz et al., 2014)[12]. The current concept of H. socolofi appears to represent a paraphyletic taxon, for example, while H. pyrrhonotus also contains distinct lineages, although further work is necessary in order to deduce taxonomic implications.
In addition to forming their own putative assemblage the bleeding heart tetras have regularly been included in the larger 'Hyphessobrycon bentosi' subgroup of the H. callistus group as proposed by Géry (1977). At the time of Géry's work H. erthythrostigma was the only recognised bleeding heart tetra and he considered it to differ from other H. bentosi subgroup members by a combination of morphometric characters plus the red humeral spot.
Weitzman & Palmer (1997) expanded Géry's concept and proposed the existence of a putatively monophyletic assemblage based on colour pattern and male fin morphology that they termed the ‘rosy tetra clade’. It was composed of species with the following shared characters which set them apart from other Hyphessobrycon species: pink to red or reddish-brown body colour; 22-29 branched anal-fin rays; 29-34 longitudinal scales; two sets of teeth on the premaxilla with 1-4 teeth in the outer series and 7-12 in the inner series.
Hyphessobrycon was raised by Durbin in Eigenmann (1908) as a subgenus of Hemigrammus, differing from the latter by the absence of scales on the caudal-fin.
The grouping was revised by Eigenmann (1918, 1921) while Géry (1977) created artificial groups of species based on colour pattern, and these definitions are still widely used today, e.g., the H. agulha group, the H. heterorhabdus group, etc. These cannot be considered to represent monophyletic assemblages, however, and their concepts continue to be redefined.
Weitzman & Palmer (1997) hypothesised the existence of a monophyletic assemblage within the genus based on colour pattern and male fin morphology that they termed the ‘rosy tetra clade’, with one of the characters supporting its monophyly being presence of a prominent dark marking on the dorsal-fin. This assemblage, plus other morphologically similar species, is considered to represent Hyphessobrycon sensu stricto by some authors, with the remaining species included in a much-expanded H. heterorhabdus group.
Others have proposed conflicting, typically more restricted, views of both the genus and/or its constituent species groups, and significant confusion remains. What is clear is that, as currently recognised, Hyphessobrycon is a polyphyletic lineage containing several genera.
The process of splitting it up has already started, and Malabarba et al. (2012) revalidated the genus Ectrepopterus Fowler, previously considered a synonym of Hyphessobrycon. They also analysed its relationships within the Characidae in the context of Mirande's (2010) previous work, but included the type species, H. compressus, for the first time in such a study. The results demonstrated that H. compressus is more closely-related to 'rosy tetra' representatives such as H. eques, H. pulchripinnis, and H. socolofi than other species then placed in the genus, including Psalidodon anisitsi, H. bifasciatus, H. elachys, H. herbertaxelrodi, and H. luetkeni.
Like many small characids, H. pyrrhonotus is susceptible to an affliction which causes some or all of the body scales to appear metallic gold in colour. It remains unclear whether this is a consequence of a parasitic infestation, as has been suggested, but affected fish appear to be in no discomfort and their lifespan and overall health seem undiminished.
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